It has been recognized for some time that the lowest friction sealing surface is not to be achieved by grinding the surface to the absolute smoothest state possible. Research, especially in the case of sealing surfaces of a rotary shaft engaged by a radial seal, has shown that a certain degree of sealing surface irregularity actually decreases the friction between the lip of the seal and the shaft. A pressure buildup upstream of the surface irregularities followed by cavitations downstream results in a net positive pressure to create a lubricant film between the seal lip and the sealing surface. This hydrodynamic film of lubricant supports the lip on the sealing surface to prevent excessive friction therebetween. These surface irregularities are left by the grinding process, generally known as a plunge grinding operation, and are in the nature of minute projections or raised asperities. However, any pattern of raised projections, regular or random, has the inherent disadvantage of interconnected valleys around such projections which work against the formation of a supporting film, by providing paths of lesser resistance that divert oil from flowing over the projections.
Research by the Batelle Memorial Institute has shown that raised asperities in the shape of right circular cylinders, arranged in a uniform pattern on a flat annular surface, will reduce seal friction by creating a hydrodynamic film between the tops of the asperities and a seal. This hydrodynamic film is created by a series of cavitations caused by the asperities. Details of the research may be found in the March, 1966 issue of the Journal of Basic Engineering, at page 177. The same article suggests that right circular cylindrical depressions would behave in essentially the same fashion to create the same hydrodynamic film. Photo etching is the process suggested for forming the depressions. Such a process, besides being expensive, produces only sharp sided or undercut depressions, and depth control is difficult. As such, it is unsuitable for the invention to be discussed.
Other configurations for a sealing surface have been found to reduce seal friction. A method for cold working or peening a flat annular sealing surface may be found in the U.S. Pat. No. 4,441,349, assigned to the assignee of the present invention. Peening of the sealing surface of a rotary shaft has been tested and reported in a February, 1981 paper, "A Friction Reducing Shaft Surface for Use With Standard Radial Sharp Lip Seals", published by General Motors. A peened surface encourages the development of a hydrodynamic, seal supporting lubricant film and does not have the inherent disadvantage of raised asperities referred to above, being a series of depressions. Such a surface is produced by either firing shot at the sealing surface, or striking it with a series of rotary flappers with shot imbedded therein. A surface so created consists of a random series of crater-like depressions, and is not capable of exact reproducibility. Besides creating a random surface, peening may be a difficult process to carry out on certain sealing surfaces which are small or inaccessible. It may also be difficult to confine the peening to a limited area.